1. Field of the Invention
The invention relates to rotary drill bits for use in drilling or coring holes in subsurface formations and of the kind comprising a bit body having a shank for connection to a drill string, a plurality of cutter assemblies mounted at the surface of the bit body, and a passage in the bit body for supplying drilling fluid to the surface of the bit body for cooling and cleaning the cutter assemblies.
2. Description of the Background
In a common form of such bit, each cutter assembly comprises a mounting body which is received in a socket in the surface of the bit body, the mounting body having a cutter portion at one end thereof. The mounting body may comprise a separately formed stud generally in the form of a cylinder of constant cross section, the cutting portion being provided by a preform cutting element mounted on a plane surface at one end of the stud which is inclined to the centerline of the stud, or is at right angles thereto.
The preform cutting element may be of the kind comprising a tablet, often circular or part-circular, having a thin, hard cutting layer of polycrystalline diamond bonded to a thicker, less hard backing layer, for example, of tungsten carbide. However, preform cutting elements are also known which consist of a unitary body of thermally stable polycrystalline diamond.
Alternatively, instead of the preform cutting element being mounted on a separately formed stud, it may be integrally formed with a backing layer of sufficient thickness for the backing layer itself to form the mounting body which is received in a socket in the bit body.
The bit body may be machined from metal, usually steel, in which case the sockets for the cutter assemblies may conveniently be machined in the surface of the bit body. In another common form of bit, the bit body, or a part thereof, is molded using a powder metallurgy process. In this case, the sockets are usually formed in the bit body during the molding process and may or may not be subject to further machining operations before the cutter assemblies are mounted on the bit body.
In either type of bit, it is necessary, in order to provide adequate strength to the mounting of the cutter assemblies in the bit body, to provide a certain minimum thickness of bit body material between adjacent sockets. Since cutter assemblies are often required to be disposed side-by-side in rows along convexly curved portions of the bit body, the inner ends of adjacent sockets are closer together than the outer ends and, consequently, it may often not be possible to arrange the cutting portions, on the projecting outer ends of the mounting bodies, as close together as is desirable.
Attempts have been made to overcome this problem by mounting preform cutting elements on studs which are noncircular in cross section. For example, by using studs which are of generally rectangular or similar cross section, the cutting elements may be packed together more closely in the bit body than when mounted on studs of circular cross section. However, the corresponding non-circular sockets may be difficult and costly to manufacture to the required accuracy. Also, in order to achieve the close packing, the thickness of bit body material between adjacent studs still requires to be small and this may be a cause of weakness in the bit.
The present invention sets out to provide an arrangement whereby adjacent cutter assemblies may be packed together closely side-by-side on the bit body, while avoiding the disadvantages of the known arrangements.